Three major types of shaft seals are known. The first principal type comprises a simple compression seal having an annular elastic body which is fitted tightly within a groove that is substantially smaller in cross-section than the body so that the body is pressed against the shaft to be sealed. Wiper-type seals are also known having a portion which is urged by a spring-steel element against the surface of the shaft. A third type comprises an elastomeric body molded around a spring-steel core which is mounted under pressure in an inwardly open groove in a housing.
In many of these seals the contact lip which rides directly on the shaft of the piston is formed of supple material which, although assuring good wiping of the shaft, is rapidly destroyed when abrasive impurities come between the lip and the shaft. Thus the service life of such seals is limited under most circumstances.
Another disadvantage of most of these arrangements is that they are unable to compensate for lateral displacement of the shaft in the housing. Thus if the shaft moves slightly from side to side as it reciprocates and/or rotates, leakage is certain to occur. Should the shaft or piston deform leakage is also a certainty.